An air distribution system should efficiently redistribute weather related unbalanced heating or cooling or high humidity conditions throughout the building in which it is installed. The currently available systems do not perform this function efficiently because of the air flow restrictions imposed by the associated duct system. In many cases, this air flow is a factor of 10 or more below that which is necessary to give acceptable performance. As a result, it often takes a forced air heating or cooling system a long time to respond to a request for a change in temperature. An efficient system should respond very rapidly to a requested change in temperature.
The hotel-motel industry has recognized the problems with central air distribution systems and has switched almost totally to individual room heat pumps. The air conditioning industry sells a large number of window units because existing central air distribution systems are costly and inadequate. The deficiency in home air circulation, especially in the basement area, has led to health and safety problems with indoor pollutants such as radon gas. The primary industry response has been the provision of high efficiency furnaces or heat pumps. These units are not worth the added expense and cannot efficiently heat the average home because an associated streamlined duct system which can provide a high air flow volume is also needed to achieve improved performance. For instance, the quoted efficiency of nearly 100 percent for the newer furnaces is measured with the furnace operating on a test stand under the ideal conditions which includes the manufacturer recommended distribution air flow volume. When that unit gets installed in an actual home where the duct system is usually inadequate, the efficiency decreases and becomes meaningless. To achieve efficiency, heat must be removed from the furnace and delivered to where it is needed. If the heat is not removed from the furnace, it will go up the chimney or the furnace will cycle on and off with associated cycling losses to degrade the efficiency.
The typical home duct system has a low air flow as the result of numerous square corners and turns in the ducts. Duct systems should be designed to be streamlined so that the air flow encounters only rounded corners. This is usually not done because of the added expense involved in producing streamlined ducts. No high efficiency heating or cooling unit can produce efficient system performance when the duct air flow is low. The supply duct and the building code required enclosed return air duct system constitute a lot of duct work that competes for space in the vicinity of the furnace and creates difficult choices for proper streamlining. The net result of all this duct work is to severely throttle the duct air distribution fan and to degrade the system efficiency.
The duct air distribution fan can create air pressure differentials much larger than the feeble flue draft. Under certain conditions, the distribution fan can completely destroy flue draft and create dangerous conditions for life and property. Building codes that require a totally enclosed return air system are the only known means to protect the relatively feeble flue draft from the pressures generated by the duct distribution fan. These code requirements are subject to many interpretations and much confusion. This results in a tacit approval for throttling the distribution fan. The throttling of this fan guarantees it will not destroy the flue draft; but it also degrades the distribution airflow volume.
The only safety device that has had some use in the past is a spillage sensor for use with gas fired appliances. Such a sensor is a thermostat switch mounted in the relief opening of a draft hood. When the flue outlet of the draft hood becomes blocked, the hot flue gasses are forced out through the relief opening and the thermostat switch is heated to its activation point and opens control power circuit to the heating appliance. Such switches are bulky and are not sensitive and a lot of flue gasses can spill before the switch trips. Furthermore, there is a substantial problem of attaching and physically securing electrical wires in a hot environment such that they are not shorted out by other metal in the vicinity. For these reasons spillage sensors are rarely used.
A more modern method of measuring available flue draft is described in U.S. Pat. No. 4,406,396. The method of this patent consists of putting a first temperature sensor, T1, inside the relief opening above the bottom of the skirt of the draft hood and putting a second temperature sensor, T2, in the air outside of and surrounding the draft hood. The temperature differential between these two sensors is related to the available draft. The two sensors have an operational transition region where the temperature differential between the two sharply increases as the flue draft goes from excessive to inadequate at the incipience of spillage. The optimum flue draft situation exists when the inner sensor T1 is approximately 15 degrees Centigrade hotter than the outer reference sensor T2. Because of the sharp rise in temperature differential as the available flue draft is decreased, the exact temperature differential is not critical and could easily be 25 degrees with equally effective results. A temperature differential of approximately 50 degrees is indicative of the onset of spillage and the heating appliance must be shut down.
It can be seen that the forced air heating and cooling systems presently available are not efficient and are inadequate because of the poorly designed duct works and duct systems associated with such systems. Efficiency is further reduced by the requirement for a separate dedicated return duct system. Since the return duct system is usually of a non-streamlined design which includes sharp corners and the like, the efficiency of the entire system is degraded.